The present invention relates to a method to generate a data cell in a telecommunication system by multiplexing a plurality of minicells into the data cell, a data cell generating arrangement able to perform this method, a data cell receiving arrangement able to communicate with the data cell generating arrangement, and a telecommunication system including such data cell generating and receiving arrangements.
Such a method to generate a data cell and related equipment are already known in the art, e.g. from working text for the planned ITU-T Recommendation I.TRUNK, entitled xe2x80x2AAL Type 2 Service Specific Convergence Sublayer (SSCS) for Trunking, published in September 1997. Indeed, ATM (Asynchronous Transfer Mode) is a well-known protocol used in telecommunication systems that is based on transmission of data in fixed size cells, the so called ATM cells. Such an ATM cell has a length of 53 octets or 53 bytes: 5 bytes form the ATM cell header whereas the remaining 48 bytes constitute a payload section that can be filled with the data to be transferred. For low bit rate applications, bandwidth utilisation of ATM can be improved by incorporation of an additional layer, the so called ATM Adaptation Layer. The ATM Adaptation Layer is based on encapsulation of data in minicells which afterwards are multiplexed in an ATM cell. The ATM Adaptation Layer can be subdivided into two parts: one or more service specific convergence sublayers (SSCS) and a common part sublayer (CPS). The service specific convergence sublayer (SSCS) serves as an interface between a telecommunication service, for instance a mobile telephony service, and the common part sublayer (CPS). Its task is to segment incoming information into packets that can be embedded in an ATM Adaptation Layer minicell. The common part sublayer (CPS) on the other hand inserts each packet in an ATM Adaptation Layer minicell, adds a 3 bytes minicell header to each minicell, and inserts the minicells in an ATM cell. The current invention is in fact dealing with the service specific convergence sublayer (SSCS). In the cited ITU-T working text, it is proposed to apply in the service specific convergence sublayer (SSCS) each speech packet separately to the common part sublayer (CPS) so that each speech packet is embedded in a separate minicell. This means that, in accordance with the known method, a minicell contains a complete data or speech packet. In narrowband applications, such as speech transmission, this known method inefficiently uses the available bandwidth capacity because of the relatively high overhead/payload ratio when applying the method of the cited ITU-T working text. Indeed, speech data packets or speech frames typically have lengths of 10 or 20 bytes. ATM Adaptation Layer minicells have a header of 3 bytes. As a consequence, in the best situation, speech frames of 20 bytes each occupy 1 minicell with a total length of 23 bytes still resulting in minicells with 15 percent overhead. This overhead percentage increases dramatically if shorter data or speech packets are inserted in minicells. Concluding, network efficiency is low if the known method is used for inserting data packets or speech packets into minicells.
It is therefore an object of the present invention to provide a method for generating data cells and related equipment but which allow to increase network efficiency in terms of bandwidth utilisation significantly.
According to the invention, this object is realised by the method for generating data cells defined by claim 1, the data cell generating arrangement defined by claim 2, the data cell receiving arrangement defined by claim 8, and the telecommunication system defined by claim 9.
Indeed, by concatenating several data or speech packets into a single minicell, the relatively large header of this minicell is shared amongst the different data or speech packets. The share of user data on the link increases whilst the share of overhead data decreases and consequently, the network efficiency improves. To enable the receiving entity to decompose a received minicell and to extract the different data or speech packets therefrom, some additional information indicative for the length of the different packets has to be added to the minicells. This overhead to be added to indicate the length of the multiplexed packets is neglectible in comparison with the overhead that is introduced in the prior art system by embedding each packet in a separate minicell.
It is to be noticed that the term xe2x80x98comprisingxe2x80x99, used in the claims, should not be interpreted as being limitative to the means listed thereafter. Thus, the scope of the expression xe2x80x98a device comprising means A and Bxe2x80x99 should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
Similarly, it is to be noticed that the term xe2x80x98coupledxe2x80x99, also used in the claims, should not be interpreted as being limitative to direct connections only. Thus, the scope of the expression xe2x80x98a device A coupled to a device Bxe2x80x99 should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.
An additional feature of the data cell generating arrangement according to the present invention is defined in claim 3.
Indeed, if the data or speech packets that will be multiplexed into minicells can have 2N different lengths which are well known in advance by the transmitting entity and the receiving entity, then the lengths of the multiplexed packets can be communicated from the transmitting entity to the receiving entity via a code of N bits per multiplexed packet. If for example 3 data packets are multiplexed into a minicell and it is supposed that data packets can have 4 different lengths, then a 2 bit code is sufficient to indicate the length of a single packet, and the transmitting entity consequently has to add 6 bits of overhead information to the minicell.
An alternative embodiment of the data cell generating arrangement which is also able to indicate the presence of empty data or speech packets, is defined by claim 4.
Indeed, via a code of N bits, the length the multiplexed packets can be indicated provided that the packets can only have 2Nxe2x88x921 different lengths or are empty. In fact, an empty packet is a packet with a length of 0 bytes so that the embodiment defined by claim 4 can be seen as a specialisation of the embodiment of claim 3 wherein one of the predefined packet lengths equals 0 bytes.
A further advantageous feature of the data cell generating arrangement according to the present invention is defined in claim 5.
Indeed, only complete bytes can be transferred in conventional telecom systems. If the number of overhead bits indicative for the lengths of the concatenated packets is not a multiple of 8, either padding bits may be added to the overhead section or alternatively some bits used for error protection of the packet length indicating bits in the data cell receiving arrangement where the data cell generating arrangement communicates with, may be added to fill the incomplete bytes.
Another additional feature of the data cell generating arrangement according to the present invention is defined by claim 6.
Thus, data packets with different origins and different destinations which have to be transferred over one and the same link somewhere in the network, may be multiplexed according to the present invention into a single minicell so that the available link capacity is not only used in an optimal way by the different users individually but is also efficiently used in a multiplexed way between the different users. What is described here is another way of grouping speech packets. Normally however, speech packets belonging to one connection will be concatenated in one minicell if the present invention is applied.
Furthermore, as is indicated by claim 7, the present invention is very suitable for implementation in an ATM (Asynchronous Transfer Mode) based environment wherein an AAL (ATM Adaptation Layer) is incorporated.